OPEN · Opening sub-ice shelf cavities and exploring their impact on dense water Production and Export in NEMO global ocean models

Opening sub-ice shelf cavities and exploring their impact on dense water Production and Export in NEMO global ocean models

Summary: Antarctic Bottom Water (AABW) is the densest and deepest water-mass in the world, formed via interactions between the atmosphere, ice sheets, sea ice and ocean. In winter in Antarctica surface waters lose their buoyancy, circulate beneath ice shelves and cascade down the continental slope to the deep ocean. AABW then circulates around the world’s oceans as the lower branch of the global ocean conveyor-belt. Perturbations in AABW properties (e.g. warming due to climate change and/or freshening from melting ice sheets) would thus have drastic consequences for global ocean properties, ecosystems and climate. The formation of the dense parent waters of AABW depends on a host of complex ice-ocean-bathymetry interactions within ice shelf cavities. At present, however, none of the Coupled Model Intercomparison Project (CMIP6) models whose projections inform the Intergovernmental Panel on Climate Change (IPCC), explicitly simulate the circulation within ice shelf cavities. Their projections neglect the feedback effects between the ice shelf and the ocean under various emission scenarios. This has resulted in a low confidence being attributed to certain projections for the Southern Ocean. Obtaining a better understanding of these processes and including sub-ice shelf cavities in global models is presently regarded as a priority. As a first step to addressing this challenge, Project OPEN has worked on opening sub-ice shelf cavity circulation under the three largest cold core ice shelves of Antarctica: Filchner Ronne, Ross and Larsen C. To do so we have used the latest version (v4.2) of the Nucleus for European Modeling of the Ocean model (NEMO) which is the ocean model for 6 of the climate groups participating in CMIP. Given the global importance of AABW, obtaining a better understanding of the dynamics of its source regions, and improving model capacity to simulate these processes is a scientific top priority. Objectives: Update the NEMO 1° global ocean configuration to open the large cold sub-ice shelf cavities responsible for dense water formation (namely Filchner-Ronne, Ross and Larsen C). We thereby allow for explicit ocean circulation within these cold cavities and keep the smaller Antarctic sub-ice shelf cavities closed for the time being. Use an adapted version of the Ice Shelf-Ocean Model Inter-comparison Experiment (ISOMIP+), named ISOMIP+K as a tool for sensitivity testing to initial conditions, various coefficients and numerical schemes. ISOMIP+K is essentially a slice of the global ocean model for each of the ice shelves of interest, and so can be used for testing but is much quicker (and cheaper) to run than the global model. Use observational data, reanalysis products and high resolution model outputs to validate NEMO global ocean configuration output in the Weddell and Ross Seas. Explore the role of sub-ice shelf cavities and the various key processes taking place on the continental shelf in determining the nature, structure and properties of dense water formation. Improve the capacity of NEMO to simulate critical dense water formation processes so as to provide new and improved configurations - fit for purpose for the next generation of coupled-climate models

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